In preparing and grinding optical lenses, for example eyeglass lenses, the primary grinding (i.e., the grinding necessary to put the primary focal or optical curvature into the lens(es) according to the eye doctor's prescription) is performed by relatively coarse abrasive materials. The result is that, after the optical curvature is ground into the lens(es), the inside (i.e., interior, concave) surface of the lens is almost opaque from grinding or scratch marks left by the optical grinding cutters.
The prevailing practice in the optometry field is to perform a "secondary" grinding or polishing operation upon those concave surfaces, to remove the grinding lines left by the primary optical cutters. This secondary polishing operation may itself comprise more than one step, but both steps are performed in substantially the same manner. Basically, the eyeglass lens is placed "scratched" (i.e., concave) side down upon a polishing "lap" or, as they are sometimes referred to, polishing master. The receiving surface of the polishing lap is convex, and is usually ground to the exact reciprocal of the concave surface of the lens, so there is virtually complete surface mating between the concave surface of the lens to be polished, and the convex surface of the lap upon which the polishing is to be accomplished.
Before the lens is placed upon the lap for polishing, the lap is provided with an abrasive polishing surface, usually in the form of a flexible adhesive-backed abrasive pad, whereby the abrasive pad is flexible enough to conform fully to the surface of the lap. The lens to be polished is impressed upon the lap (with adhesive pad thereon) in such a way that the surface of the lens to be ground is in full contact with the polishing pad upon the lap.
The combined lens-lap-pad is placed onto a polishing machine and the assembly is caused to oscillated or vibrated in the presence of a polishing lubricant and in such a way that the polishing pad and lubricant removes the grinding marks left by the primary optical cutters. Often the polishing is accomplished in conjunction with providing some sort of a mildly abrasive slurry, of approximately the same grit as is applied to the polishing pad, to the lap-pad-lens assembly while the polishing is taking place.
In two stage polishing processes, such as the polishing process utilized by this inventor, after the slurry polishing step is completed, the first polishing pad is removed, and a second, finer (i.e., less gritty, less abrasive) pad is placed upon the lap. the lens reimpressed thereupon, and the lap-pad-lens assembly is reoscillated/vibrated, this time usually with only water as the provided solution (although other solvents, even including a minor grit solvent, could be used).
It is generally known and appreciated that the laps must be contoured very precisely, to the inverse curvature of the optometric Prescription, so the convex surface of the lap will coincide exactly with the concave surface of the optical lens being polished. This is accomplished on commercial lap cutters which are generally known in the art.
Because of the relatively high cost of machining and the time it takes to machine present laps, laps are presently provided in large number of different starting shapes (as many as 72 lap shapes), shapes which are provided in an estimate of the final ground contour, so as to minimize the amount of lap material which must be machined off to exactly match the prescription contour In addition, often it is necessary to have more than one set of laps for each pair of glasses, one set for glass lenses, and a second set for plastic lenses. As you might imagine, the inventory of laps in many optical labs is very high.
In addition, even though presently available laps are made of relatively machineable metal (i.e., aluminum; see infra), it still consumes quite a bit of time to grind a lap.
The techniques and procedures just above described are well known and understood in the optical grinding art.
The standard lap that is used in the optical lab industry is made of aluminum, which replaced cast iron as the lap of choice some ten years ago. However, even though aluminum laps provided advantages over cast iron laps because of their noncorrosiveness, relative ease of machineablity and relative (i.e., compared to cast iron, at any rate) low weight, the aluminum laps in use today have the nagging disadvantage of high machine time to grind to the reciprocal contour of the prescription (as much as 6-12 minutes per lap), and their still relatively high weight (about 12 ounces) causes wear and tear on the polishing apparatus. In addition, the time delay caused by multiple passes on the lap cutter tempts the operator to try to machine off too much aluminum in a single pass, and this often results in chipping of the aluminum, commonly called "dinks." If a lap becomes "dinked", it must be thrown away.
For many years, the industry has striven to provide a plastic lap to replace and avoid the problems associated with the use of aluminum laps, but without success--until now.
Attempts to provide a viable plastic lap have heretofore failed, principally for two reasons. First, while it is well known to use injection molding to produce easily machinable plastic parts, standard injection molding techniques do not produce an acceptably strong lap structure to withstand the mechanical stresses and abuses of the polishing process. This is because, ordinarily, injection molded plastic parts have only a relatively thin "skin" or "shell" of solid plastic to form the upper lap surface, and the structural integrity (such as it is) of the lap is provided by the standard "webbing" practice well known in the injection molding arts.
The other well known plastic molding process, compression molding, has proven unacceptable because the plastics which are utilizable in compression molding (i.e., thermal set plastics, such as phenolics) are generally brittle, and would crack or chip when attempting to machine them to required contour.
The present invention overcomes these problems and provides an acceptable plastic lap through a combination of a unique material selection and a molding process which departs substantially from the prevailing teachings ordinarily associated with injection molding. The particular material utilized is a mineral-filled thermoplastic polymer, which is injection molded to its final shape into a super-heated mold via an inlet valve or gate which is much, much larger than ordinarily necessary, and also utilizing process parameters that exceed the prevailing published process and design criteria for the material, and can only be accomplished by an injection molding press that is about three times the size and capacity that would ordinarily be called for to injection mold a part of the size and shape of standard optical laps.